2 Homometallic Alkoxides

Homometallic Alkoxides 37
2.8 Transesterification Reactions of Metal Alkoxides (Method H)
As early as 1938, Baker showed that aluminium alkoxides undergo facile transesterification
reactions, which can be represented by Eq. (2.94):
Al(OR) 3 C 3CH3COOR 0 ⇀
↽ Al(OR 0 ⊳3 C 3CH3COOR " ⊲2.94⊳
where R 0 D a primary or secondary alkyl group.
He also observed that the reactions of Al(OEt)3 with CH3COOBu t yielded only the
heteroleptic alkoxide, Al(OEt)(OBu t )2.
Mehrotra 293 in 1954 confirmed Baker’s observation and extended the above procedure
for the preparation 321 of higher alkoxides of titanium, zirconium, and hafnium;
the alcoholysis of the convenient starting alkoxide, Zr(OPr i )4.Pr i OH, with Bu t OH was
extremely slow owing to the small difference in the boiling points (in parentheses) of
Pr i OH (82 Ž C) and Bu t OH (83 Ž C), whereas the much larger difference in the boiling
points of CH3COOPr i (89 Ž C) and CH3COOBu t (98 Ž C) facilitated the reaction considerably,
providing the first convenient method for the preparation of Zr(OBu t ⊳4:
Zr(OPr i ⊳4.Pr i OH C 4CH3COOBu ↽ ⇀ Zr(OBu t ⊳4 C Pr i OH C 4CH3COOPr i
⊲2.95⊳
The technique has also been successfully employed for the preparation of heteroleptic
alkoxides by carrying out the reaction(s) in the desired stoichiometric ratio in the
solvent cyclohexane (b.p. 81ŽC), which forms a convenient lower boiling azeotrope
with CH3COOPri (Eq. 2.96):
t cyclohexane
Zr(OPr i ⊳4.Pr i OH C xCH3COOBu t
cyclohexane
↽ ⇀ Zr(OPr i ⊳4 x ⊲OBu t ⊳x C Pr i OH C xCH3COOPr i
⊲2.96⊳
The method was extended by Mehrotra and co-workers for the preparation of tertiary
alkoxides of a number of metals: lanthanides, 142,145,146,150 titanium, 321 hafnium, 321
vanadium, 322 niobium, 323 tantalum, 324 iron, 283 and gallium. 184,185
Following the transesterification technique, Bradley and Thomas 325,326 extended the
technique for a more convenient preparation of trialkylsiloxides of titanium, zirconium,
and other metals:
M(OPr i ⊳4 C 4R3SiOCOCH3 ! M(OSiR 3⊳4 C 4CH3COOPr i " ⊲2.97⊳
Transesterification reactions have the following advantages over alcoholysis reactions:
(a) The tert-butoxide derivatives of elements can be easily prepared from the corresponding
isopropoxides as there is a significant difference in the boiling points
of their organic esters (¾9 Ž C) compared with the corresponding small difference
in the boiling points of the two alcohols (¾0.2 Ž C); this makes the fractionation
of the more volatile ester much easier.
(b) In some cases the esters (e.g. silyl acetate) are much more stable than the
corresponding alcohols (silanol), which sometimes undergo self-condensation